Among living primates, humans are most closely related to the apes, including chimpanzees, gorillas, orangutans and gibbons, but little is known about their last common ancestor.
"We've got many spectacular fossil finds showing how humans evolved in Africa over the last six to seven million years, since our lineage split from that of the chimpanzees," said Fred Spoor, professor at University College London in the UK.
"Until now, relevant fossils are scarce and incomplete which is why this new discovery is extremely important for furthering our knowledge," he said.
The ape fossil, nicknamed Alesi by its discoverers, and known by its museum number KNM-NP 59050, comes from a critical time period in the African past.
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It is the most complete known in the fossil record and was spotted in 13 million year-old rock layers in the Napudet area, west of Lake Turkana in northern Kenya, by Kenyan fossil hunter John Ekusi in 2014.
"Nyanzapithecus alesi was part of a group of primates that existed in Africa for over 10 million years. What the discovery of Alesi shows is that this group was close to the origin of living apes and humans and that this origin was African," said Isaiah Nengo, of the Turkana Basin Institute.